The present invention relates to the manufacture of optical fibers in general, and more particularly to a method of and an apparatus for making solid cross-section optical preforms with a cladding/core ratio in a predetermined range from tubular bodies.
There are already known various methods of, and arrangements for, producing optical fibers, most of which involve first the production of an optical preform and then the drawing of the optical fiber from the fiber. One way of producing the optical preform is the modified chemical vapor deposition method, during the preformance of which a gaseous medium is caused to flow through the interior of a tubular substrate body. The gaseous medium includes reactants which decompose or interreact or are otherwise chemically transformed when heated to transformation temperatures into solid glass constituents. The tubular substrate is locally heated from the outside and the heating zone is gradually moved longitudinally and/or circumferentially of the tubular substrate, resulting in transformation of the reactants into the glass constituents, precipitation of the glass constituents from the gaseous medium in the form of glass soot and deposition of such glass soot on the internal surface of the tubular substrate and subsequent fusing of the deposited glass soot with the substrate and/or any previously deposited internal layers. The modified chemical vapor deposition process is conducted in a plurality of passes or increments. The composition of the gaseous medium can be changed from one pass to another so as to change the chemical composition and the optical properties of the consecutively deposited layers of glass. After the deposition operation is completed, the resulting glass formation, which is still tubular, is often caused to collapse into a solid cross-section optical glass preform, which is subsequently used in the optical fiber drawing operation.
The cross-sectional areas of the substrates and/or the cross-sectional areas of the core layers of the tubular formations differ from one tubular formation to another, so that the ratios of such areas are also different for different tubular formations. This means that, unless special measures are taken, the ratio of the cladding region to the core region would also differ from one optical preform to another, such difference reflecting itself in a similar difference in the cladding/core ratio of the fibers drawn from such optical preforms. This is very disadvantageous, especially since such ratio should be kept within a very close range to assure that the optical fiber will have the desired optical properties.
To deal with this problem and to assure that the cladding/core ratio will be within the desired range for all optical fibers, it has been heretofore proposed to measure the cross-sectional area and/or wall thickness of the tubular substrate, and to control the deposition in the interior of the tubular substrate in accordance with the obtained results so as to achieve a core/cladding ratio in the resulting tubular formation that will eventually result in the desired core/cladding ratio in the optical preform and eventually in the optical fiber drawn from the latter. Yet, this procedure, which involves careful selection of the tubular substrates, careful control of the deposition process, and the maintenance of a considerable stock of tubular substrates to meet the expected needs for optical fiber preforms to be manufactured under given conditions leaves much to be desired.